Four coal slag samples from commercial suppliers were tested for fibrogenicity in male Sprague-Dawley-rats. The geometric mean diameters for Slag-I, Slag-II, Slag-III, Slag-IV, and Minusil (a positive control) were 0.47, 0.62, 0.60, 0.56 and 0.41 micrometers, respectively. The four slag samples were found to be chemically similar. The rats received a single intratracheal instillation of 40 milligrams of test material. Sterile water was instilled in the vehicle control group. Lung function decrements were determined by intubation of anesthetized rats and measurements of pulmonary function parameters such as flow, volume, transpulmonary pressures, and ventilatory performance. The rats were killed at 2 days, 3, 6, or 12 months for the determination of treatment related histopathological lesions in various tissues. The Minusil group had significantly lower body weights at 1 to 12 months than the Slag-I, Slag-II, and Slag-IV group, and at 2 to 5 months than the Slag-III group. The Minusil group had significantly greater average lung weight and hydroxyproline content than the slag treated groups. The mean weights of Slag-II and Slag-III groups were significantly greater than the vehicle control group. Statistical analyses showed that the average lung and tracheobronchial lymph nodes (TBLN) dust burden and dust retention were significantly greater for the Minusil group than all slag groups. The average TBLN dust burden and dust retention were significantly greater for Slag-IV than for Slag II- and-III groups. No significant differences were seen in pulmonary function analyses between the slag groups and vehicle control group. The Minusil group had significantly lower lung volumes and flow rates. Histopathological examination found mild to moderate interstitial fibrosis in the slag treated groups, which increased in severity with time. Differences between slag groups were minor in comparison to the Minusil treated group. The authors conclude that coal slags have reduced toxicity in comparison to Minusil.